1. Field of the Invention
The present invention relates to a driving circuit for a DC brushless fan motor, and more particularly to a DC brushless fan motor driving circuit having low electromagnetic noise.
2. Description of Related Art
Two types of driving circuit exist to drive a DC brushless fan motor that has a single coil (52). One type is a single-phase full-wave motor driving circuit, and the other one type is a two-phase half-wave motor driving circuit. Each motor driving circuit uses a driver element to drive the fan motor, and the output driving signal of the driver is a form of pulse width modulation (PWM).
With reference to FIG. 4, one DC brushless fan motor driving circuit has a PWM driver (50), a Smith trigger (53), a Hall sensor (not shown), a lock detecting and auto-re-start circuit unit (54), an oscillator (501) and a full-bridge switching circuit unit (51).
The PWM driver (50) has a Hall signal input terminal (I1), a PWM input terminal (I4), a low speed setting terminal (I5), a frequency output terminal (FG O/P), a reset terminal (I3), a clock input terminal (I2) connected to the oscillator (501) and multiple controlling output terminals (O1˜O4). Each controlling output terminal (O1˜O4) outputs a controlling signal, that is a form of a PWM wave signal. The low speed setting terminal (I5) is connected to an external low speed setting signal (Low Speed Set). The PWM input terminal (I4) is connected to an external PWM input signal (PWM I/P). The external PWM input signal (PWM I/P) adjusts the speed of the fan motor by changing the width of the PWM input signal.
The Hall sensor is connected to the hall signal input terminal (I1) of the PWM driver (50) through the Smith trigger (53). The Smith trigger (53) converts the Hall sine wave signal output from the Hall sensor to a square wave signal.
The lock detecting and auto-re-start circuit (54) has two inputs and one output. The two inputs are respectively connected to the Smith trigger (53) and the oscillator (501), and the output is connected to the reset terminal (I3) of the PWM driver (50).
The full-bridge switching circuit (51) can have four electronic switches (not shown), each of which has a controlling terminal. The four controlling terminals are connected respectively to the output terminals (O1˜O4) of the PWM driver (50) and the coil (52) of the fan motor. The PWM driver (50) controls the conductive sequence of the full-bridge switching circuit (51) and the conductive period of each electronic switch to adjust coil current amplitude and a coil current direction in the coil (52) of the fan motor.
The PWM driver (50) compares the PWM input signal (PWM I/P) and the low speed setting signal (Low Speed Set) to determine whether the PWM input signal (PWM I/P) is larger than the low speed setting terminal (Low Speed Set). If the PWM input signal (PWM I/P) is larger than the low speed setting signal, the PWM driver (50) will drive the fan motor at a higher speed through the full bridge switching circuit (51). If not, the PWM driver (50) will keep the fan motor at a low speed condition.
With further reference to FIGS. 5A to 5C, three different controlling signals (COT1˜COT3) are input to one controlling terminal (O1) of the full bridge switching circuit (51) to produce three coil currents (IL1˜IL3). Since the controlling signal is formed of the PWM wave signal, the first to third controlling signal (COT1˜COT3) are different pulse widths respectively with 20%, 50% and 80% duty cycles. When the fan motor is driven by the third controlling signal (COT3) having an 80% duty cycle, the speed of the fan motor is faster than the fan motor driven by the first or second controlling signals (COT1, COT2). However, each coil current (IL1˜IL3) has a serious ripper phenomenon and the rate of change of the coil current is large. Therefore, the fan motor driven by the forgoing controlling signals will be electromagnetically noisy. In addition, the reliability of the motor driving circuit will be reduced.
With reference to FIG. 6A, forming the controlling signal from a PWM wave signal is easy, but electromagnetic noise is not reduced or eliminated. Since the controlling signal is formed from a PWM wave signal similar to a square wave signal, the rate of change of the coil current is large, and the ripper phenomenon is obvious.
To overcome the forgoing drawback, one solution is proposed. With reference to FIG. 6B, forming the controlling signal as a trapezoidal wave signal makes the rate of change of the coil current smaller than that of the coil current driven by the PWM wave signal. Thus, the electromagnetic noise will be slightly reduced. Although the trapezoidal wave signal slightly reduces the electromagnetic noise, the frequency of the trapezoidal wave signal is limited in a specific range. Therefore, to achieve the same high speed of the fan motor, a motor driving circuit that is implemented to output the trapezoidal wave signal requires a larger current than that of the PWM driving circuit. For example, to drive the fan motor to work at 4000 rpm, the motor driving circuit with the PWM controlling signal requires a coil current of about 300 mA, and the motor driving circuit with the trapezoidal wave signal requires about 350 mA. Therefore, the motor driving circuit with the trapezoidal wave signal has higher power consumption.
Therefore, the present invention provides a DC brushless fan motor driving circuit to reduce the electromagnetic noise and consume less power.